In early May 2021, several variants of the SARS-CoV-2 coronavirus were circulating in the U.S., with the alpha strain dominant by far. In June, the delta strain began spreading more quickly, and by the beginning of September was responsible for over 99% of all genetically sequenced COVID-19 cases in the U.S. As of Dec. 4, 2021, omicron had not yet made major inroads against delta.